1. Field of the Invention
The present invention relates to a fuel cell having an electrolyte electrode assembly including an anode, a cathode, and an electrolyte interposed between the anode and the cathode. The electrolyte electrode assembly is interposed between separators.
2. Description of the Related Art
Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly. The electrolyte electrode assembly is interposed between separators (bipolar plates), and the electrolyte electrode assembly and the separators make up a unit of fuel cell for generating electricity. A predetermined number of fuel cells are stacked together to form a fuel cell stack.
In the fuel cell, an oxygen-containing gas or air is supplied to the cathode. The oxygen in the oxygen-containing gas is ionized at the interface between the anode and the electrolyte, and the oxygen ions (O2−) move toward the anode through the electrolyte. A fuel gas such as hydrogen-containing gas or CO is supplied to the anode. Oxygen ions react with the hydrogen in the hydrogen-containing gas to produce H2O or react with CO to produce CO2. Electrons released in the reaction flow through an external circuit to the cathode, creating a DC electric current.
For example, Japanese laid-open patent publication No. 2002-203579 discloses a solid oxide fuel cell shown in FIG. 19. As shown FIG. 19, the solid oxide fuel cell is formed by stacking power generation cells 1 and separators 2 alternately. Each of the power generation cells 1 includes a fuel electrode layer 1b, an air electrode layer 1c, and a solid electrolyte layer 1a interposed between the fuel electrode layer 1b and the air electrode layer 1c. A porous conductive fuel electrode current collector 3 is provided on one surface of the power generation cell 1, and a porous conductive air electrode current collector 4 is provided on the other surface of the power generation cell 1. The fuel electrode current collector 3, the power generation cell 1, and the air electrode current collector 4 are sandwiched between a pair of separators 2.
The separator 2 has a fuel gas supply passage 5 and an air supply passage 6. The fuel gas supply passage 5 is connected to a fuel gas hole 5a formed at a substantially central region on one surface of the separator 2. The air supply passage 6 is connected to an air hole 6a formed at a substantially central region on the other surface of the separator 2. The fuel gas hole 5a faces the fuel electrode current collector 3. The air hole 6a faces the air electrode current collector 4.
The fuel gas such as H2 or CO flows through the fuel gas supply passage 5, and is from the substantially central region of the separator 2 toward the center of the fuel electrode current collector 3. The fuel gas flows through holes formed in the fuel electrode current collector 3 toward the substantially central region of the fuel electrode layer 1b. Then, the fuel gas flows along unillustrated slits to move radially outwardly toward the outer region of the fuel electrode layer 1b. 
Likewise, the air is supplied from the substantially central region of the separator 2 toward the center of the air electrode current collector 4. The air flows through holes formed in the air electrode current collector 4 toward the substantially central region of the air electrode layer 1c. Then, the air flows along unillustrated slits to move radially outwardly toward the outer region of the air electrode layer 1c. In this manner, in each of the power generation cells 1, the fuel gas is supplied to the surface of the fuel electrode layer 1b, and the air is supplied to the surface of the air electrode layer 1c to carry out power generation.
According to the disclosure of Japanese laid-open patent publication No. 2002-203579, the fuel gas flows outwardly from the substantially central region to the outer region of the fuel electrode layer 1b, and the air flows outwardly from the substantially central region to the outer region of the air electrode layer 1c. The unreacted fuel gas and air are mixed together to cause reaction around the outer region of the power generation cell 1. After the reaction, the remaining fuel gas and air are discharged as an exhaust gas. The amount of air supplied to the power generation cell 1 tends to be excessive in contrast to the amount of fuel gas supplied to the power generation cell 1. Therefore, substantial amount of oxygen is present in the exhaust gas. The outer region of the power generation cell is likely to be exposed to the oxygen.
The fuel electrode layer 1b is made of metal such as nickel (Ni). The metal (Ni) of the outer region of the fuel electrode layer 1b is oxidized undesirably into NiO by the exposure to the oxygen. Further, the exhaust gas containing the oxygen flows in the fuel electrode current collector 3, and reduction reaction of NiO is prevented. NiO has a high electrical resistance. Therefore, the effective surface area used for power generation is reduced by the presence of NiO. Consequently, the desired power generation performance (efficiency) of the power generation cell 1 can not be achieved.